The invention relates to a scrambling and descrambling method in a system for transmitting and receiving a scrambled television signal comprising an analog video signal and digital sound data transmitted in frequency-division multiplex, in which the analog video signal is transmitted in lines and fields whose starting instants are controlled by a first time base, and the digital sound data are transmitted in the form of successive packets, parts of which are transmitted in an unscrambled form and other parts are transmitted in a scrambled form, the rate of the data being controlled by a second time base which is not synchronous with that for the video signal, while control words used for scrambling and descrambling are changed in the course of time and, when the control word for the video signal is to be changed, the new control word is first determined and transmitted and subsequently a command of using the new word is transmitted to the video part.
The invention also relates to a television transmitter and a receiver for transmitting or receiving signals in accordance with this method.
A television scrambling method as described in the opening paragraph is known from the document EP-A-0 448 534. This document describes a means for scrambling a sound signal in accordance with the NICAM standard. The document does not suggest any means for scrambling the video signal in a corresponding manner.
Based on known means for scrambling a sound signal and other known means for scrambling a video signal, which means are completely independent of each other, the idea of the invention is to simplify the system by a common implementation of certain elements of the scrambling devices.
The invention is based on the recognition that separate pseudo-random generators can nevertheless be initialized with the same control word, provided that particular precautions are taken for the synchronization of the devices.